Weigh bed

ABSTRACT

A weigh bed has a rectangular base frame, a rectangular weight frame mounted above the base frame, a load cell beam mounted on each corner of the weigh frame and a ball connecting each load cell beam to the base frame. Three tie rods connected between the base frame and weigh frame limit horizontal movement of the weigh frame with respect to the base frame.

This is a division of application Ser. No. 07/371,611, filed June 26,1989.

This invention relates to a weigh scale, and more particularly, to aweigh scale associated with a hospital bed.

BACKGROUND OF THE INVENTION

In scales generally, it is conventional to mount a weigh frame above abase and rigidly connect load cells between the weigh frame and thebase. As subject matter to be weighed is placed on the weigh frame, thestressing of the load cells, through the application of the weight,causes the load cells to generate an electrical signal proportional tothe weight applied to the load cells. The loads measured by respectiveload cells are summed to provide the measure of the weight of thesubject matter mounted on the scale.

The rigid or substantially rigid connection between the base and theweigh frame is satisfactory where the base is fixed with respect toground. But where the scale is to be moved from place to place on aregular basis, or where the load is applied in diverse directions orattitudes, thereby introducing force vectors that are not parallel tothe force of gravity, the rigid mounting of the weigh frame with respectto the base frame will introduce errors. Any twist of the weigh framewith respect to the base frame will cause binding at the load cell. Thefrictional force of the binding will be seen as weight that is eitheradded to or subtracted from the true weight, but in any event, willintroduce error.

A hospital bed having provision for weighing a patient is an example ofthe type of scale environment that causes error. This is particularlytrue in relation to special critical care-type beds that are leased forlimited periods, returned, and leased again for use at another site. Thefrequent movement of the bed from one site to another compounds theproblems referred to above. A ground support that is not essentiallyperpendicular to the force of gravity introduces a twist of the weighframe with respect to the base frame causing error. The position of thepatient--for example, consider a patient in an inclined trendelenburgposition--changes the vector of the load on the load cell. Any changefrom a strictly vertical application of the load that is, parallel tothe force of gravity, introduces error.

BRIEF SUMMARY OF THE INVENTION

It has been an objective of the present invention to provide a scalestructure, particularly useful in a hospital bed, that minimizes error.Whereas in conventional scales an error of 1 pound in 5,000 isconsidered to be satisfactory, the present invention limits error toabout 1 pound in 20,000. The error is held to a minimum notwithstandingthe surface on which the bed is mounted or the deviation from horizontalof the attitude of the patient on the bed.

The objective of the present invention is attained by connecting a weighframe to a base by four load cells, one at each corner of the base. Eachload cell is fixed to the weigh frame and connected to the base frame bymeans of balls. Each ball engages a plane surface on the base. Thefour-ball contact provides assurance that if the patient is supported atan angle to the weigh frame, or if the base is supported at an angleother than horizontal, there will be no twist imparted to the load cellswhich would introduce an error into the combined reading of the loadcells.

But, the weigh frame must be laterally confined with respect to the baseframe, for otherwise the ball contact would permit the weigh frame toroll off the base frame when the bed is in transport or when the bed isat an angle to horizontal. This problem is solved by providing athree-tie rod connection between the base frame and the weigh frame.Preferably, two tie rods are mounted across each opposite end of the bedand one tie rod is mounted on one side of the bed parallel to thelongitudinal axis of the bed. The three-tie rod configuration greatlyfacilitates assembly. In this respect, a loose analogy to a four-leggedstool versus a three-legged stool is appropriate. The three-legged stoolis easily leveled to the ground without rocking. A four-legged stoolrequires very precise measurement and assembly of the legs to assurethat the four ends of the legs lie in the same plane. Otherwise, thefour-legged stool will rock. So it is with three tie rods. Four tie rodswould appear to be the obvious approach. Four tie rods require greattime and care in adjustment to avoid stressing the load cells. A threetie rod connection is much simpler to install and maintain and hasproved to provide the requisite security.

BRIEF DESCRIPTION OF THE DRAWINGS

The several objectives and features of the present invention will becomemore readily apparent from the following detailed description taken inconjunction with the accompanying drawings in which:

FIG. 1 is a side elevational view of the bed including the weigh frame;

FIG. 2 is a disassembled, perspective view of the weigh frame and baseframe; and

FIG. 3 is a fragmentary cross-sectional view of one load cell connectionbetween weigh frame and base frame.

DETAILED DESCRIPTION OF THE DRAWINGS

A bed 10 is shown in FIG. 1, the bed having an air mattress 11 by whicha patient 12 is supported. The bed 10 includes an articulated patientsupport 15 which is mounted on a bed frame 16. The bed frame 16 isconnected by parallelogram lift arms 17 to the lower bed structure 18.All of the mechanism, including drive motors and the like for operatingthe bed and manipulationg the patient support panels to change theattitude of the patient, are mounted on the bed frame 16. That mechanismincludes the motor mechanism for raising and lowering the bed frame 16with respect to the lower structure 18. The lift arms 17 connect the bedframe and superstructure to the weigh frame 20 which forms a part of thelower structure 18.

Each lift arm 17 is fixed to a shaft 19. Each shaft 19 is fixed at oneend to an L-lever 21. The other end of the L-lever has a nut 22 throughwhich a screw 23 passes. The screw is rotated by a motor in a housing 24to raise and lower the bed frame 16.

Rollers 26 are mounted on the lower ends of the lift arms 17. Therollers are mounted in horizontal slots 27 and 28 at the head and footends of the base frame 29. Slots 28 facilitate shifting the bed frame toa trendelenburg position. Slots 27 accommodate any slight warping of theframe as the bed is raised and lowered. Such warping would, but for theslots, cause a binding that affects the weight reading.

Preferably, a U-shaped bracket 25 is fixed between the lower ends of thelift arms 17 to allow adequate side-to-side motion of the left arms toprevent binding of the frame, while at the same time controlling theside-to-side motion so that the rollers never fall out of the supportingbrackets.

The weigh frame 20 is connected to a base frame 29 only by four loadcells, as will be described. Thus, it can be seen that all of thesuperstructure, weighing approximately 700 pounds, is supported abovethe weigh frame 20. Any change of the angle of the patient's body withrespect to the bed frame will not affect at all the force of the loadcells on the base frame because of the ball connection of weigh frame tobase frame, as will be described. The base frame 29 is mounted oncasters 30 so that the bed can easily be rolled from place to place.That transport of the bed of course introduces the possibility of errorwhich is accommodated by the manner of mounting of the weigh frame tothe base frame.

The mounting of the weigh frame 20 to the base frame is best illustratedin FIG. 2. The base frame has two longitudinal members 31 and 32 whichare joined together at the center by a transverse brace 33. Rectangular,tubular cross members 34 and 35 are fixed across the ends of thelongitudinal members 31 and 32 to provide a secure rectangular baseframe that will not twist horizontally. The tubular members 34 and 35can be welded or bolted to lugs 36 at the ends of the longitudinal framemembers.

The weigh frame 20 is likewise a rectangular member having twolongitudinal members 37. Intermediate their ends, the longitudinalmembers 37 are recessed as at 38 to accommodate the central brace 33when the weigh frame is nested within the base frame. Rectangular tubes39 and 40 are welded across the ends of the longitudinal members 37 toprovide a rigid rectangular weigh frame structure.

The weigh frame is mounted on the base frame by means of load cells 41at each corner of the weigh and base frames. The load cells areidentical to each other. One of the load cells is best illustrated inFIG. 3. The load cell includes a rectangular beam 45 on which straingauges 46 are mounted. All strain gauges are electrically connected byconductors 47 to a digital weight indicator 48. The weight indicator canbe of the type disclosed in copending application Ser. No. 07/138,829,filed Dec. 28, 1987.

Each beam 45 is bolted to the inboard end 49 to the weigh frame 20 by apair of bolts 50. The bolts sandwich a spacer 51 between the beam 45 andthe weigh frame 20. The tubular end members 34, 35 of the base frame 29have openings 55 through which the spacer 51 passes, thereby connectingthe weigh frame 20 to the load cells 41 free of any contact with thebase frame 29.

The other, outboard, end 60 of the beam 45 has a bore or recess 61. Ahardened steel ball 62 is freely positioned in the bore 61. About a0.005 inch clearance is provided between ball and bore to facilitateslight rolling. A threaded seat 64 is threaded into a boss 65 in thetubular end members 34, 35 of the base frame. The seat 64 has a hardenedsurface 66 which is engaged by the ball 62. The ball 62 provides what isessentially a rolling contact with the surface 66. Visually, it can beobserved that the ball rolls a small fraction of an inch in alldirections around the hardened surface 66 of the seat 64.

An overload stop is created by the weigh frame and base frame surfacesforming gap 67 at each corner of the frames. A vertical motion stop isformed by screw 68 threaded in the end 60 of the beam 45 and contactingball 62. The size of vertical motion gap 69 between screw 68 and crossmember 34, 35 may be varied by rotation of the threaded seat 64. Closinggap 69 opens overload gap 67. It is preferred to set overload gap 67 toabout 200% overload capacity or about 1,000 pounds at each corner sothat the weigh frame will bottom out on the base frame when the load atany corner exceeds about 1000 pounds.

Since the rolling ball contact is the only contact between the weighframe and the base frame, a system of tie rods is required to keep theweigh frame from moving horizontally with respect to the base frame. Tothis end, an end tie rod 70 is connected across one end between the baseframe at 71 and the weigh frame at 72. At the opposite end, a tie rod 73is connected to the corner of the base frame at 74 which is at the sameside of the bed as the connection at 71 of tie rod 70. Tie rod 73 isconnected to the weigh frame at 75. These "same side" connectors reducetwisting of the weigh frame with respect to the base frame. Finally, alongitudinal tie rod 77 is connected between a flange 78 on the baseframe at bolt hole 79 and the weigh frame at 80. Rubber transport stops81 are provided as cushioning between the superstructure and the weighframe during transportation of the bed.

In operation, the bed 10 can be moved from place to place. The tie rods70, 73 and 77 will keep the weigh frame securely in position on the baseframe, or more precisely, will keep the balls 62 at the four corners ofthe weigh frame continuously resting on the seats 64 of the base frame.The ball support of the weigh frame provides freedom of motion on thetwo horizontal axes. The balls isolate all forces from the load cellexcept gravity. Each ball maintains the point of loading constant and,hence, a constant lever formed by the beam 45.

If the base frame is not perfectly level with respect to horizontal,there will be no binding of the connection of the weigh frame withrespect to the base frame because of the pure ball connection betweenthe two. Similarly, if the patient's position is shifted as, forexample, to a trendelenburg position or a reverse trendelenburgposition, there will be no effect whatsoever on the accuracy of theweight transmitted by the load cells since all force vectors will beresolved into vertical force vectors by the ball connections to the ballseats between the load beam and the base frame.

From the above disclosure of the general principles of the presentinvention and the preceding detailed description of a preferredembodiment, those skilled in the art will readily comprehend the variousmodifications to which the present invention is susceptible. Therefore,we desire to be limited only by the scope of the following claims andequivalents thereof.

We claim:
 1. In a hospital bed having a base frame,casters supportingsaid base frame above ground, a bed, a linkage below said bed connectedto said bed to support said bed for vertical movement with respect tosaid base frame, power means for raising and lowering said bed, and aweigh frame mounted solely by means of load cells to said base frame,said linkage and power means being mounted on said weigh frame wherebysaid linkage and power means support said bed on said weigh frame andsaid weigh frame is mounted on said base frame only by load cells.